Simple power display panel addon

[ncitro]

Well without a schematic of the electrical I don't know which leg is pulling what. One thing is for sure, there is no way I was pulling 44 amps out of a single leg and still maintaining 122 volts and not popping a breaker. At the 30AMP site the meter showed only 113 volts with 21 amps being drawn as in the original post picture. That one is with a Camco dog bone adapter.

Would you say you had approximately the same things running on both the 30 amp and 50 amp site? The dogbone adapter will not make power disappear, so if you were running the same loads then the readout should be close at both sites, not double regardless of what was on which line. If you have more running (dual AC units or Electric Water Heater) then that would make sense.

howson That was exactly my point. I do not see how it could be working reading only the neutral. When he reported it was, I assumed there was something I was missing (see my comment above about my physics being a while ago).

[howson]

ncitro -- as you probably know by now it helps (me) to draw stuff out to try and understand what's going on...

[Jlawles2]

OK, from my knowledge of electricity i got from my dad. 120v needs one hot and one neutral. The neutral even though shared by the out of phase hots still must carry the amperage of the 2 hot legs back to the transformer otherwise you get 240 volts. the meter is doing its job because its only measuring the amperage total going through the line. the amperage does not cancel out across the neutral other wise you would have 240 volts. Now that would be bad. If the neutral did not need to carry the full amperage of each leg, then it could be much smaller, however RV's pull up to 30 amps per leg and the next size plug is 50 amps, so they went that way to protect the neutral circuit.

[ncitro]

OK, from my knowledge of electricity i got from my dad. 120v needs one hot and one neutral. The neutral even though shared by the out of phase hots still must carry the amperage of the 2 hot legs back to the transformer otherwise you get 240 volts. the meter is doing its job because its only measuring the amperage total going through the line. the amperage does not cancel out across the neutral other wise you would have 240 volts. Now that would be bad. If the neutral did not need to carry the full amperage of each leg, then it could be much smaller, however RV's pull up to 30 amps per leg and the next size plug is 50 amps, so they went that way to protect the neutral circuit.

I think this is incorrect. There is 240v between the two hots in an RV and 120v between either hot and the neutral. The two hots are out of phase and the current on the neutral should subtract, however it does need to be sized the same as the hots in case there happens to be no load on one of the hots. A 50 amp RV is capable of pulling up to 50 amps on each leg for a total of 100 amps of 120v (this rarely if ever occurs).

[Jlawles2]

By chance did you check the voltage between the 2 hot legs? I wonder if the campground was setup with both of the hots on the same leg? Stranger things have happened.

Also remember AC voltage is measured RMS and the net result is always positive.

[Livin Large]

Would you say you had approximately the same things running on both the 30 amp and 50 amp site? The dogbone adapter will not make power disappear, so if you were running the same loads then the readout should be close at both sites, not double regardless of what was on which line. If you have more running (dual AC units or Electric Water Heater) then that would make sense.

howson That was exactly my point. I do not see how it could be working reading only the neutral. When he reported it was, I assumed there was something I was missing (see my comment above about my physics being a while ago).

When I try to run more than the 30 amp post will handle I get as far as 35 amps reading before the breaker pops.

[gbkims]

A sample of a multiwire branch circuit from Mlke Holt helps me visualize how Neutral current on 50A Legs with out of phase voltage and Neutral current with a dogbone supplying both legs in the RV. from a 30A outlet adds.
http://www.mikeholt.com/instructor2/...3728sample.pdf

[howson]

A sample of a multiwire branch circuit from Mlke Holt helps me visualize how Neutral current on 50A Legs with out of phase voltage and Neutral current with a dogbone supplying both legs in the RV. from a 30A outlet adds.
http://www.mikeholt.com/instructor2/...3728sample.pdf

Base on Mike Holt's diagram, I think what I sketched together is right! I'd post a pic, but his drawings are copyrighted so must refrain.

[TedS]

https://www.rvtravel.com/rv-electric...her-explained/

I've linked a Mike Sokol article. He notes in a paragraph that the neutral currents from both hot legs should be subtractive, otherwise you could get 100 amps in the neutral at full load. That's more than the neutral should carry.

Edit: I copied this from another Sokol article about why a neutral was burnt when connected to mis-wired pedestal.

The hat trick is that when this 50-amp connector is fed by two different legs (poles) the current in the neutral subtracts rather than adds. So if you’re drawing 40 amps of current on the Hot 1 leg, and perhaps 30 amps of current on the Hot 2 leg, then instead of the expected 70 amps of current on the neutral (40 + 30 = 70), you only have 10 amps of current on the neutral (40 – 30 = 10). And that’s why the neutral wire and contacts in your shore power cord are sized to handle a maximum of 50 amps of current rather than 100 amps of current. If you force a 50-amp rated wire and connector to carry 70, 80 or 90 amperes of current, then it will overheat and eventually burn up.

[ncitro]

https://www.rvtravel.com/rv-electric...her-explained/

I've linked a Mike Sokol article. He notes in a paragraph that the neutral currents from both hot legs should be subtractive, otherwise you could get 100 amps in the neutral at full load. That's more than the neutral should carry.

Edit: I copied this from another Sokol article about why a neutral was burnt when connected to mis-wired pedestal.

The hat trick is that when this 50-amp connector is fed by two different legs (poles) the current in the neutral subtracts rather than adds. So if you’re drawing 40 amps of current on the Hot 1 leg, and perhaps 30 amps of current on the Hot 2 leg, then instead of the expected 70 amps of current on the neutral (40 + 30 = 70), you only have 10 amps of current on the neutral (40 – 30 = 10). And that’s why the neutral wire and contacts in your shore power cord are sized to handle a maximum of 50 amps of current rather than 100 amps of current. If you force a 50-amp rated wire and connector to carry 70, 80 or 90 amperes of current, then it will overheat and eventually burn up.

This is how I’ve always understood it.

[Livin Large]

By chance did you check the voltage between the 2 hot legs? I wonder if the campground was setup with both of the hots on the same leg? Stranger things have happened.

Also remember AC voltage is measured RMS and the net result is always positive.

I seriously doubt it. I have a hard wired Surge Guard 35550 between the post and the breaker panel. If the post isn't wired correctly it triggers a fault code and prevents power from entering the RV. My AC current sensor is between the neutral output of the Surge Guard and the input of the panel.

[Country Campers]

Spending the day with Randy Davis yesterday at Grand Design , Randy is a seasoned Tech for GD who teaches all of their techs as well as weekly sessions with dealer techs , the amps measured thru the white wire is subtractive. This is why on a hardwired EMS the coils are located on the 2 "hot" legs to measure the amp draw on each leg. Power measured between each leg should be 240 volts ac , between each leg and neutral white should be 120 volts ac , between each leg and the ground wire should be 0 volts ac.
Just thought that I would pass this on.

Brian

[ncitro]

Spending the day with Randy Davis yesterday at Grand Design , Randy is a seasoned Tech for GD who teaches all of their techs as well as weekly sessions with dealer techs , the amps measured thru the white wire is subtractive. This is why on a hardwired EMS the coils are located on the 2 "hot" legs to measure the amp draw on each leg. Power measured between each leg should be 240 volts ac , between each leg and neutral white should be 120 volts ac , between each leg and the ground wire should be 0 volts ac.
Just thought that I would pass this on.

Brian

Thanks Brian, this tracks with my understanding of how it should be working. Thank you for the clarification.

[Livin Large]

So all that being said and I don't doubt what you're all saying, it all makes perfect sense. Question is does it make any difference leaving the sensor on neutral as opposed to L1 for the purpose of monitoring a 30amp hookup? Also how did I get a 44 amp draw show up on the display from all those things running at the same time at a 50 amp site if it subtracts? No dog bone was used at this campground.

[Livin Large]

The reason it works is because the meter measure the current on the neutral not the hot(s). If you look at a 50 amp to 30 amp adapter, the 2 hot legs are joined on a single phase. This works because a camper runs on 110v returning everything through the neutral. If anything in the unit ran on 220v, it would not pick up that current as all of the current runs through the hot legs only and not the neutral.

No adapter was used at the 50 amp site.